Securing Jerry Cans to Panzer III

Almost all of those racks were fabricated in the field. Of course they used what ever was available. Most of them were a sheet metal rack with either a strap or strap and bar arrangement. Check out Squadron's "In Action" series, you'll see many different, easy to reproduce racks. Shep Paine's book shows a few as well.

Spamicus offers some very good options in his post. Check your resources or the net for any pics on the Panzer III (and believe me there's lots out there) and look at them to determine what method was used. Some had racks, some straps, some were places in wooden boxes and others were just tied to the vehicle.

If you don't want to go the scratchbuilding route, Verlinden makes (or made) a p/e set of brass rails and stowage to build the racks seen on the back of Stug's and Panzer III's. I don't think it's OOP and builds up rather nicely.